I owned a 2008 500 UCE for 5 years, 15k miles and now have a 2017 500 which I've put 3k on. I have ridden a Meteor and a Classic but only on hour long test rides however I had the pleasure of following a friend last week (I was riding my Himalayan) for over 1400 miles on our John O Groats to LE ride.
I'll start of by saying that Enfield have done a superb job of capturing the looks, feel and sound of the the 500 with the new 350 and it's a very worthy successor, I'm a big fan. IMO it's a better bike than the 500 in every way apart from, for me, one important area.
I'll get that out of the way first of all, for my use which is 75% two up 50 - 150 mile days out with lots of B and C roads including some steep hills, it just hasn't got enough power and torque. It pulls very nicely from low down but, solo (I'm 17st in my kit so no lightweight), starts to run out of puff 55- 60 although it will pull past an indicated 70mph if no headwind on the flat it takes a lot longer than the 500 to get past 70. Two up (my wife is an 8st gym bunny fortunately) it doesn't seem much slower at low speeds but more, important than being able to get over 60mph, is that it struggles much more on some of the very steep hills. It's about 25% down on bhp & torque over the 500 and uphill you can really tell.
Having said all that, the motor is so much smoother (yet still has a lovely thump) than the 500 and it feels like it will stand being flogged all day, a bit like the Himmy does, whereas we all know that a 500 just doesn't feel great trying to rev the wotsits of it to maintain speed or overtake. If headwind and hills allow, cruising at 50 - 60mph on the 350 is much nicer than on the 500.
Handling and braking are similar but just that bit better, comfort is also better, helped by minimal vibes but also by two superb seats. Riding position is, for me at least, again a little better as the footrests seem slightly further back (can't be sure of that) and the handlebar is a little higher without that curious Bullet bend. Again, no massive difference but what there is is better not worse.
Where there is a huge difference is in perceived quality of just about everything you can see and touch (and if you could be bothered, probably taste and smell) as well . Special mention to the switchgear, levers and motor castings, they shame many more expensive bikes and aren't just good for the price. The whole thing just seems cleaner and more cohesive as well, for example the engine area is far less cluttered, fewer stray cables, the efi unit is far less ugly, no evidence of that silver heat resistant paint so beloved of the 500 production line bods.
The icing on the cake is the sound it makes, better than the hideous standard exhaust on the 500 and, despite my best efforts with after market item and baffles, even slightly better (again IMO) than I've achieved on my 500.
To sum up, I would trade my 500 for a new 350 in the blink of an eye if only it had a bit more grunt but if you mainly ride solo, don't try to overtake things over 50mph without a very long straight, and don't mine going a bit slower up steep hills then go for one, they're luvvvverly